In January 1974, in the middle of the last great economic crisis, the critic Raymond Williams was watching a discussion on television when he heard the presenter say: "Well, finally – and I'm sorry to have to hurry you on this but we're almost out of time, so can you answer very briefly, in fact you have just 20 seconds – is Britain really on the edge of disaster?" Williams contrasted the relentlessness of the clock in these "serious" discussions with sports programmes where there was "an endless dribble of words, a kind of relentless filling-in, as if television were timeless".

The Williams rule, that the time devoted to a television discussion rises in relation to a subject's unimportance, holds even truer today. If you want an extended debate about Ivana Trump's earlobes, you can find it on the digital channels, where Celebrity Big Brother and other reality shows have their follow-up programmes. Here, because there is so much time to fill, the talk seems to go on and on, or maybe it is just that time passes more slowly. Meanwhile, on a recent Newsnight 30th anniversary special, Martin Amis, AS Byatt, Chris Patten and others were each given just a few minutes to discuss social and political change in Britain over the last three decades – a subject that one might have thought would merit at least as much time as celebrity earlobes.

This situation probably has more to do with the generic conventions of television than our addiction to trivia. All broadcast talk has to fight against its own artifice. Hilda Matheson, the BBC's first director of talks, more or less invented the protocol in the early 1930s. At a time when most radio talk was in the form of a monologue, she believed it was "useless to address the microphone as if it were a public meeting" and devised the interview format as a way of avoiding the "parsonical drone" of the more boring speakers. Most radio discussions before the war, though, were scripted and rehearsed, with differences of opinion acknowledged through the exchange of prepared statements. Even today, some of this artificiality survives in any televised discussion: the participants have to conduct a public conversation as if the public is not watching, and to pretend that the discussion is happening in real time rather than in the unreal, constrained time of television.

In the early days of television, it was quite acceptable for discussion programmes to finish later or earlier than scheduled. That inspired creator of comic gobbledegook, Professor Stanley Unwin, was popular with producers precisely because if a show was under-running he could talk nonsense to fill the gap. Now the schedules are more tyrannical, and serious programmes seem to fear boring viewers by dragging an item on for too long, so the presenters have people pestering them via their earpieces to move on to something else.

A lot of TV discussions have a death wish: they are driven, paradoxically, by the desire to bring the conversation to an end. Instead of developing organically they look for the definitive answer to a problem or argument, so they can reach that habitual cut-off moment when the presenter says "I'm afraid we've run out of time", or "we'll have to leave it there". I wonder if the pleasure of Test Match Special, with its longueurs and digressions, has less to do with the cricket than the rare experience of overhearing an uninterrupted conversation that unfolds according to its own logic.

I have a vision of the eco-apocalypse in the year 2110. Half of Britain is under water and people have retreated to the hills, where the water is lapping round their ankles. On every TV channel they are rambling on about reality show trivia as if time were endless – except for one serious discussion on BBC4, where the presenter is chivvying along a panel of distinguished experts: "I'm going to have to hurry you because we're almost out of time but … are we all doomed?"